FIG. 1 is a diagram showing signal waveforms in such a track jumping operation which may be obtained by using, e.g., an integrated circuit M51562P for a pickup servo system, which is of a kind produced by Mitsubishi Electric Corporation. An explanation of these signal waveforms is shown at page 11 of data sheets for Mitsubishi Linear Integrated Circuit M51562P. In the diagram, a waveform (a) denotes a drive current flowing through a tracking actuator coil. A waveform (b) indicates a tracking sensor signal which is obtained by an output signal from a photodetector provided in an optical pickup device. A waveform (c) denotes an output signal from a comparator which receives the tracking sensor signal (b) and a predetermined reference signal. A waveform (d) indicates a track jump command signal for determining the duration of a track jumping operation. A waveform (e) represents an acceleration pulse signal for a tracking actuator. A waveform (f) denotes a deceleration pulse signal for the tracking actuator.
The track jumping operation of a conventional optical disc system will now be described with reference to FIG. 1. First, the acceleration pulse signal (e) for the tracking actuator is generated simultaneously with the leading edge of the track jump command signal (d) which has been output from a microcomputer in the optical disc system. Then, the tracking servo loop is opened, so that a difference signal between the acceleration pulse signal (e) and the deceleration pulse signal (f) is applied to a tracking actuator drive circuit through a gain amplifier. Thus, an acceleration current flows through a tracking actuator coil to thereby drive the tracking actuator. As the light spot traverses a track, a track traverse signal is caused as the tracking sensor signal (b). When the light spot has traversed the half track, the tracking sensor signal (b) crosses the zero level. This zero crossing point is detected by the leading edge of the comparator output signal (c) of the tracking sensor signal (b), so that the acceleration pulse signal (e) of the tracking actuator is completed and, at the same time, the deceleration pulse signal (f) is generated.
At this time, a deceleration current in the direction opposite to that of the acceleration current flows through the tracking actuator drive coil. The tracking actuator continuously moves in the same direction as that upon acceleration while its speed is reduced. When the speed of the tracking actuator approaches zero due to this deceleration, the light spot reaches the center between the adjacent tracks and the tracking sensor signal (b) again crosses the zero level. When this zero crossing is detected by the comparator output signal (c) of the tracking sensor signal (b), the deceleration pulse signal (f) of the tracking actuator is terminated. The tracking jump command signal (d) is also completed at this time and the tracking servo loop is closed. Thus, the track jumping operation is completed.
According to the foregoing conventional optical disc system, the switching from the acceleration pulse signal to the deceleration pulse signal is performed solely on the basis of the detection of the zero crossing point of the output signal of the photodetector. This leads to the following problems. When the tracking sensor signal fluctuates due to disturbance caused by any defect on an optical disc medium, a vibration caused by a disc motor, or the like, the zero crossing point appears before the movement of the half track of the light spot (i.e., in the case of an erroneous detection), so the acceleration time becomes too short and the deceleration time becomes long. As a result, the light spot cannot reach the adjacent track and the track jumping operation fails.